1. Field of the Invention
The present invention relates generally to the field of positive displacement (PD) type internal combustion engines (ICE), and more specifically relates to a shunt pulsation trap for improving its thermodynamic cycle efficiency and for reducing exhaust gas pulsation and induced noise, vibration and harshness (NVH) from such internal combustion engines.
2. Description of the Prior Art
An internal combustion engine (ICE), also commonly known as a piston engine is a mechanical device in which the combustion of a fuel causes the expansion of gas under high temperature and high pressure in order to move piston and generate useful mechanical work. For almost 130 years, piston type ICE have found widespread use in almost every facet of life as a power source, especially for mobile applications, such as tractors, automobiles, boats and small aircrafts, or as household power tools of lawn mowers, trimmer, etc. There are generally two types, gasoline engine (Otto Cycle) and diesel engine (Diesel Cycle) based on fuel and cycle, or 4-stroke and 2-stroke based on strokes needed for one cycle, or reciprocating and rotary based on motion. ICEs are known for high exhaust noises they generate, ranging from 100-180 dB at cylinder discharge if not silenced, well beyond the Permissible Exposure Limit of 75 dB set by National Institute for Occupational Safety and Health (NIOSH) or the 90 dB of the Occupational Safety and Health Administration (OSHA). It is becoming more and more a nuisance to the surrounding environment with people living nearby or working in office.
A PD type ICE generally converts the energy of burned gases to shaft power through either linear or rotary positive displacement movement. As classified in FIG. 2a, piston motion can be either a 4-stroke or 2-stroke fueled by either gasoline or diesel, while rotary motion is exemplified by a Wankel engine. No matter what type of motion, shape, number of strokes or fuels (which in a broader sense includes other fuels such as kitchen grease), it undergoes the same thermodynamic Otto or Diesel cycle by trapping a fixed amount of fuel and gas into a cavity, burning that gas, expanding the gas to do mechanical work and discharging the consumed gas to outlet or atmosphere. Structurally, they all have in common an intake port, a fuel delivery device, a varying volume cavity, a discharge port and valves controlling the timing of the intake and discharge release of gas mixture. FIG. 3a shows the common phases and stroke function of a conventional 4-stroke piston engine: intake, compression, combustion, expansion and discharge. FIG. 3b shows the actual Otto cycle and its ideally simplified thermodynamic process as an isobaric inlet, isentropic compression, isochoric combustion, an isentropic expansion and an isochoric plus isobaric discharge. FIG. 3c shows a typical cycle coupled with an exhaust muffling operation and FIG. 3d the generic structure of a piston 25 inside a cylinder 20 forming a cavity 37 with a discharge port 38 connected to a catalytic converter 3 and an outlet muffler 5 in series.
In operation, the inlet stroke of the IC engine sucks in the vaporized fuel mixture (or just air in a diesel engine) into the cylinder as the piston moves to the maximum volume position BDC (bottom dead center) shown as 0→1 in FIG. 3a-b, and the inlet valve closes. In compression stroke, both valves are closed and the piston starts its movement to the minimum volume position known as TDC (top dead center) and compresses the fuel mixture. During this approximated isentropic process as shown by 1→2 in cycle diagram, the pressure, temperature and density of the fuel mixture all increase. It is immediately followed by an almost instantaneous combustion simplified as an isochoric process 2→3, as the fuel mixture burns. The increased high pressure exerts a greater amount of force on the piston and pushes it towards the BDC. Expansion of working fluid takes place isentropically from 3→4 and work is done by the system that is transmitted to the crank shaft. At the end of the power stroke the exhaust valve suddenly opens, releasing heat and some gas instantly as shown as a constant volume or isochoric process from 4→1. The exhaust stroke follows when piston starts its movement from BDC to TCD or 1→0, exhausting the rest of gas mixtures from the cylinder. At the end of this stroke, the exhaust valve closes, the inlet valve opens, and the sequence repeats in the next cycle.
4-stroke ICEs require two revolutions for a complete cycle while 2-stroke engines only need one revolution to complete the almost same process as described above. Many petrol and gas engines work on a cycle which is a slight modification of the Otto cycle such as Diesel cycle that uses a compression heating ignition rather than a separate ignition system. This variation allows diesel fuel be injected directly into the cylinder so that combustion occurs at constant pressure, instead of a constant volume as in Otto cycle. Another variation is called Atkinson and Miller cycle that has an asymmetrical compression and expansion strokes by using variable valve timing so that it operates more fuel efficiently.
In essence, all PD engines divide the continuous inlet gas stream mechanically into parcels of cavity size that is then disposed discretely to the exhaust after the work is done. This process inherently generates gas pulsations with a low frequency, often called cylinder firing rate, which is equal to discharge valve opening frequency or RPM/60 for a 2-stroke and RPM/120 for a 4-stroke ICE. On the other hand, the discharge pulsation is very significant if the cavity gas pressure is higher than the discharge exhaust pressure at the moment the discharge valve opens. It is this pressure difference at the discharge opening that is responsible for generating large amplitude pulsations which in turn excite noises and vibration of the entire engine system. The pulsations generated by the pressure difference at sudden discharge valve opening are periodical pressure spikes as high as 100-180 dB which could cause downstream mechanical damages and high noise if not properly controlled. Most often, pulsations are confined within the exhaust system and could result in fatigue failures of downstream components such as cylinder head, exhaust manifold, catalytic converter and exhaust muffler, or the turbocharger if equipped.
The muffler is now often required at the engine exhaust in order to control the exhaust pulsation and noises. The most common muffler type is reactive or sometimes absorptive or a combination of both is used too. This serially connected muffler or silencer is generally very effective in pulsation and noise control, reducing noise level (sound pressure level) by as much as 20-55 dB, but it suffers a fair amount of pressure losses at the same time. In principle for a serial muffler, more effective noise attenuation is always at the expense of higher pressure loss. Sometimes pressure loss can be as high as 2-4 psi under high load conditions. The loss of pressure or the increased exhaust back pressure reduces engine mechanical power output hence affecting its fuel efficiency. It is one of the contributing factors that results in low engine efficiency today, just 20-30%. An example for this trade-off is found on some performance vehicles such as race cars that used to replace a higher-attenuating-higher-loss reactive type muffler with a less-attenuating-less-loss absorptive type muffler. But the ever stringent regulations from the government and growing public awareness of the global warming and comfort level in residential and office areas have reversed that practice and given rise to an urgent need for both quieter and more efficient internal combustion engines as exemplified by new emission standards as CARB (California Air Resources Board).
The present invention tries to meet these environmental needs and tackle the problems at the source of loss and noise from a different perspective. The underlining theory is based on a postulation that large amplitude waves and instantaneous flows induced by pressure difference at the moment of exhaust valve opening are the primary cause of discharge pulsations and noises. This theory is analogous to a well studied physical phenomenon as occurs in a shock tube (invented in 1899) where a diaphragm separating a region of high-pressure gas from a region of low-pressure gas inside a closed tube suddenly burst open. As shown in FIGS. 1a-1b, when the diaphragm is suddenly broke open, a series of expansion waves is generated propagating from the low-pressure to the high-pressure region at the speed of sound, and simultaneously a series of pressure waves which can quickly coalesces into a shockwave is propagating from the high-pressure to the low-pressure region at a speed faster than the speed of sound, and inducing rapid fluid flow behind the wave front. An interface, also referred to as the contact surface that separates low and high pressure gases, follows at the same fluid velocity after the pressure waves or the shock wave.
To understand pulsation generation mechanism in light of the shock tube theory, let's review a cycle of a classical 4-stroke engine as illustrated in FIGS. 3a-3d by following one flow cavity from inlet to exhaust. First the vaporized fuel and air mixture enters into the cavity formed by a piston and a cylinder as inlet valve opens and closes. After the cavity is closed to both the inlet and outlet, the trapped gas is then being compressed, ignited and expanded, driving the piston doing mechanical work. When a desired expansion ratio is reached, the cavity is suddenly opened to the outlet and discharged. A serially connected discharge muffler is there to attenuate pulsation and noise generated at the exhaust opening.
In general, the cavity pressure of an ICE is higher than the outlet pressure as exhaust gas needs to get out of the cavity. This results in a forward flow rushing out of the cavity to equalize the outlet pressure as soon as the cavity is opened to the discharge according to the conventional theory. Since this happens almost instantaneously and there is almost no volume change for the cavity, the expansion is regarded as a constant volume process, or isochoric process as shown as 4→1 in FIGS. 3a-3b. However, according to the shock tube theory, the discharge valve opening at point 4 as shown in FIGS. 3a-3b, resembling the diaphragm bursting of a shock tube as shown in FIG. 1b, would generate a series of expansion waves into the cavity. The fan of expansion waves would sweep through the high pressure gas inside the cavity and expand it at the same time at the speed of sound. This results in an almost instantaneous adiabatic expansion because wave travels much faster than the fluid particle or piston. In this view, the wave induced expansion is the primary driver for pressure equalization process from 4→1 and this process is adiabatic in principle other than the conventionally assumed isochoric.
In view of the new theory to explain the pulsation generation mechanism, as the expansion waves travel to high pressure cavity as shown in FIG. 3d, simultaneously generated pressure waves or a shock wave front travel in the opposite direction causing rapid pressure increase and inducing forward flow down-stream. This shock wave travelling downstream at a speed faster than the speed of sound and inducing a fast flow behind is the dominant source of discharge pulsation and noise for a positive displacement internal combustion engine. Any effective pulsation and noise control should target this high velocity large amplitude mixture of waves and induced flow while minimizing the main flow losses at the same time.
Since the amplitude of gas pulsation in ICE is typically much higher than the upper limit of 140 dB set in the classical acoustics, the small disturbance assumption or the resulting linear wave equation is often inadequate to predict its behavior. Instead, the following rules can be used for large disturbances when the SPL is beyond 140 dB. These rules are based on the above discussed Shock Tube theory and can be used to judge the source of gas pulsation and quantitatively predict its amplitude and travel directions. In principle, these rules are applicable to different gases and for gas pulsations generated by any industrial PD type gas machinery or devices such as engines, expanders, or pressure compressors, vacuum pumps and valves.                1. Rule I: For two divided compartments (either moving or stationery) with different gas pressures p4 and p1 (FIG. 1a), there will be no or little gas pulsations generated if the two compartments stay divided;        2. Rule II: If the divider between the high pressure gas p4 and the low pressure gas p1 is suddenly removed (FIG. 1b), gas pulsations are generated at the location and moment of the opening as a composition of a fan of Pressure Waves (PW) or a quasi-shock wave, a fan of Expansion Waves (EW) and an Induced Fluid Flow (IFF) with magnitudes as follows:PW=p2−p1  (1)EW=p3−p2  (2)ΔU=(p2−p1)/(p1×W)  (3)Where ρ1 is the gas density at low pressure region, W the speed of the lead pressure wave, ΔU the velocity of Induced Fluid Flow (IFF);        
3. Rule III: Pulsation PW is the action by the high pressure gas p3 to the low pressure gas p1 while pulsation EW is the reaction by the low pressure gas p1 to the high pressure gas p3 in the opposite direction, and their magnitudes are such that they equally divide the initial pressure ratio p3/p1 [equation (4): p2/p1=p3/p2=(p3/p1)1/2]. At the same time, both PW and EW pulsations induce a unidirectional fluid flow pulsation IFF in the same direction as the PW.
Rule I implies that there would be no or little pulsations during compression (expansion) and combustion phases of a ICE cycle because of the absence of either a pressure difference or sudden opening. The focus instead should be placed upon the intake and exhaust phases, especially at the moment of the intake and discharge when it is suddenly opened and when there is a pressure difference at the opening.
Rule II indicates specifically the moment of pulsation generation as the instant the divider separating p3 and p1 opens and the location as the divider. Moreover, it defines two sufficient conditions for gas pulsation generation:
a) The existence of a pressure difference Δp31;
b) The sudden opening of the divider separating that pressure difference.
Because all PD gas machinery converts energy between shaft and fluid by dividing incoming continuous fluid stream into parcels of cavity size and then discharging each cavity separately at the end of each cycle, there always exists a “sudden” opening at discharge phase to return the discrete parcels back to a continuous stream again. So both sufficient conditions are satisfied at the moment of discharge opening if there exists a pressure difference between the cavity and outlet it is opened to. For ICE, this pressure difference is always existing as ΔP41 as shown in P-V diagram of FIG. 3. In addition to the pressure difference induced pulsation, there co-exists a hardware (like a valve) induced flow pulsation too, but its magnitude is typically much smaller for most existing fluid machinery, and is roughly proportional to its equivalent velocity pressure. FIG. 2b shows graphically the relationship between the initial unbalanced pressures and the summation of resulting gas pulsations.
Rule II also reveals the composition and magnitudes of gas pulsations as a combination of large amplitude Pressure Waves (PW) or a quasi-shockwave, a fan of Expansion Waves (EW) and an Induced Fluid Flow (ΔU). These waves are non-linear waves with changing wave form during propagation. This is in direct contrast to the acoustic waves that are linear and wave fronts stay the same and do not induce a mean through flow. It is interesting to note the wholeness of three different pulsations (PW, EW and IFF) that are generated simultaneously and one cannot be produced without the others. This makes gas pulsations very difficult to control because it's not one but all three effects have to be dealt with.
Rule III shows further the interactions between two gases of different pressures are mutual so that for every PW pulsation, there is always an equal but opposite EW pulsation in terms of pressure ratio (p2/p1=P3/p2). Together, they induce a unidirectional fluid flow pulsation (IFF) in the same direction as the pressure waves (PW).
It should also be emphasized the drastic difference on magnitude and behavior between acoustic waves and pulsations discussed above. First of all, the linear acoustics is limited to pressure fluctuation levels below 140 dB, equivalent to pressure below 0.002 Bar or 0.03 psi. For industrial type fluid machinery, the measured pressure fluctuation or pulsation is often in the range of 0.3-30 psi (or even higher), equivalent to 160-200 dB according to the SPL definition. So the pulsation pressures inside the industrial fluid machines are much higher and well beyond the pressure range intended in the Classical Acoustics. Physically, the acoustic waves are sound waves travelling at the speed of sound with no macro fluid movement with it while pulsations are a mixture of strong pressure and expansion waves that also induce an equally strong macro fluid flow travelling with speeds from a few centimeters per second up to 1.89 times of the speed of sound (Mach Number=1.89), for example. It is this large pressure forces and induced high velocity fluid flow that could directly damage a system and components on its travelling path, in addition to exciting vibrations and noises. With the above proposed Pulsation Rules, it is hoped that more realistic pulsation prediction is made possible so that the true nature of pulsations can be realized, hence controlled.
Accordingly, it is always desirable to provide a new design and construction of a PD type ICE that achieves high pulsation and NVH reduction at source, improves fuel efficiency and eliminates the discharge muffler at the same time.